Orthogonal frequency division multiplexing (OFDM) transmission technique applied in high-speed wireless networks is very popular, and the OFDM transmission technique can resist frequency selective fading and interference between symbols effectively. The advantages of OFDM transmission technique that have made this technique popular in wireless systems are sometimes counterbalanced by one major problem of a very high peak-to-average power (PAPR), so that a power amplifier requires a very large linear operated area due to the high PAPR, and a serious signal distortion may result easily. Alternatively, a power amplifier with a relatively larger linear operated area is required. However, power amplifiers of this sort are generally high priced.
There are two main conventional methods of reducing the PAPR, respectively: a signal distortion technique and a distortionless PAPR reduction technique. The principle of the signal distortion technique attempts to reduce an amplitude of a sample node having a too-large signal power in order to achieve the PAPR reduction effect. For example, a clipping method directly restricts an input signal of a very large amplitude to be transmitted within a predetermined range of values. However, the signal amplitude is damaged easily, such that the signal spectra will be aliased to cause the issue of in-band distortions. On the other hand, the distortionless PAPR reduction technique does not require any signal distortion technique, and “A Comparison of Peak Power Reduction Schemes for OFDM” authored by S. H. Muller and J. B. Huber published in IEEE Global Telecommunications Conference, GLOBALCOM '97, Phoenix, Ariz., pp. 1-5, November 1997 discloses a partial transmit sequence (PTS) method, which is publicly accepted as one of the methods capable of reducing PAPR effectively. Since the PTS method involves linear operations, there will be no destructive interference to the signals of the OFDM transmission technique. Without considering noises, a receiving end can demodulate the signal completely. However, the PTS algorithm involves a high level of complexity and a huge computation capacity, and thus the PTS method is not cost-effective.